An Experimental Study of an FEL Oscillator with a Linear Taper Page: 3 of 18
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enhance the efficiency of FELs. The theory of the tapered wiggler FEL  assumed a
single frequency, plane wave input and was thus applicable to amplifiers. The phase
displacement scheme  also assumed a plane wave, though pulsed effects were
considered. The two schemes share one common feature. They assume a well-defined
bucket as an initial condition. However, the small-signal gain at the optimal saturated
frequency was smaller than the saturated gain, so oscillators had a startup problem .
The first tapered oscillator therefore used a multi-component design that enhanced the
small signal gain at the appropriate frequency and produced a good trapping fraction .
Later, high efficiency (up to 45%) was achieved in an amplifier experiment at Livermore
. In 1995, experimental studies at FELIX and Orsay  and theoretical studies by
Saldin, Schneidmiller, and Yurkov  showed that a mild negative taper should produce
better extraction efficiency than a positive taper. The authors in ref.  showed that there
was a mismatch between the optimal frequency for small signal gain and the optimal
frequency for saturated lasing for the case of a positive tapered wiggler, leading to poor
performance for mildly tapered FEL oscillators. There is no such mismatch for a
moderately inverse tapered oscillator, allowing it to operate more efficiently.
In many experiments, inserting a wedge in the wiggler gap, leaving the untapered
gap at either the front or back end, provides the taper. In this case, the ratio of the small
signal gain of the tapered wiggler to that of the untapered wiggler is independent of
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Benson, S.; Gubeli, J. & Neil, G.R. An Experimental Study of an FEL Oscillator with a Linear Taper, article, January 1, 2001; Newport News, Virginia. (digital.library.unt.edu/ark:/67531/metadc725065/m1/3/: accessed November 18, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.